Projectiles fired from the barrel of guns are propelled by a highly pressured gas discharge produced from the burning of a propellent. While the projectile must be free enough in the barrel to be accelerated to a maximum velocity, it must effectively form a gas seal with the bore so as to extract the maximum available energy from the expanding gas. Most commonly the bore includes rifling to spin the projectile and the rifling may tend to strip off some amount of lead from the projectile as it heats and travels along the barrel at very high velocity.
It is popular for firearms or small arm owners to home cast projectiles using a two stage process. Firstly, a nominally sized lead slug is cast. Secondly the slug is pressed through an accurately sized die and a copper seal is simultaneously press fitted to its base. The finished projectile normally includes annular driving bands and, intermediate the driving bands, wax grooves. The wax is forced into the grooves during the pressing operation and, when fired, reduces leading of the barrel.